A multi-valent pneumococcal polysaccharide vaccine, is composed of many types of Streptococcus pneumoniae capsular polysaccharide. Normally, these polysaccharides are produced in conventional fermentation. The pneumococcal polysaccharides are separated from other fermentation products by art-recognized methods such as lysing, centrifugation, alcohol fractionation, and ammonium sulfate precipitation. These steps are followed by lyophilization and other purification steps unique to each pneumococcal type. As immunogenicity of the pneumococcal polysaccharides appears to be directly proportional to molecular weight of the polysaccharide (i.e. higher molecular weight equals higher immunogenicity), it is desirable that the pneumococcal capsular polysaccharides be maintained as the largest possible polymers with such polymers retaining the native state structure. This will result in a more effective immunogenic response for a vaccinated subject.
The Bureau of Biologics of the Food and Drug Administration has established standards for the apparent size of all types of pneumococcal polysaccharide polymers used in vaccines. In accordance with their standards, all polysaccharides in present vaccines must have a Kd. value on a Sepharose.RTM. 4B column of less than a range 0.15 to 0.35 (depending on the type of polysaccharide) where Kd. is defined as the ##EQU1## With Type-19 pneumococcus, the Kd. limit is 0.25 which corresponds to molecular weight of approximately 500,000.
Many of the pneumococcal capsular polysaccharides are sufficiently stable such that the integrity of the molecules are maintained throughout the process of purification of the polysaccharides. Type-19 is exceptional in displaying instability during lyophilization. The Type-19 molecule composed of L-rhamnose, D-glucose, 2-acetamido-2-deoxy-D-mannose and phosphate tends to depolymerize producing molecules with molecular weights below acceptable Kd standards. There is some suggestion that the presence of the phosphate moiety contributes to this observed instability of the polysaccharide.